1. Technical Field
The invention relates to sandwich panels and, particularly, to sandwich panels utilizing precured reinforced cores and a method of manufacturing the same.
Sandwich panels are used in a wide variety of applications requiring structural and/or thermal insulation properties. These applications include structural and non-structural uses in rapid transit vehicles, refrigerated and non-refrigerated buildings, boats, aircraft, recreational vehicles, enclosed trailers and many others. Structural sandwich panels are composite structures formed by bonding two generally thin facings or skins to a relatively thick core material. The skins, which are normally dense and strong, resist compression and tension, while the core, which is normally made of relatively weak and low-density material, serves to separate the skins, stabilize them against buckling and resist shear loads.
2. Related Prior Art
Flooring assemblies used for mass transit vehicles are an example of a simple type of sandwich panel. Common materials used for the construction of flooring panels are varieties of wood and stainless steel. It is generally known to construct such flooring assemblies by providing thin sheets of stainless steel, which are affixed over the top of a support frame made of wood, usually balsa or plywood. The steel sheet provides an exposed surface having toughness and durability, while the wood frame provides the flooring with a lightweight, rigid supporting structure. This combination of components and material has become well-known in the industry as being desirable because certain constructions have the capacity to exceed safety requirements, including flame and smoke tests, which are required for all flooring assemblies used in mass transit applications.
Common cores for more standard sandwich panels are rigid expanded plastic foams and honeycomb materials. Honeycomb core usually comprises a thin sheet material, such as paper or aluminum foil, which is formed into a variety of cellular configurations. Expanded plastic foam cores usually provide much higher levels of thermal insulation than honeycomb, but honeycomb cores are normally substantially stronger than insulating foam cores of comparable density.
Various methods of introducing insulating foams into the cells of honeycomb have been used for the purpose of filling the voids or adding higher levels of thermal insulation to structurally adequate honeycomb core. These include such approaches as applying foaming chemicals to the honeycomb cells, and pressing slabs of plastic foam into the cells. However, these processes are difficult to perform in thick core sections, limit the types of foams that can be used to fill the cells of the honeycomb uniformly, or require large capital investment in machinery. As a result, such composite cores have enjoyed little use in most sandwich panel applications, and many honeycomb core products are consequently deficient in insulation and subject to migration of water into the core.
Sandwich panels with skins of metal, wood, fiberglass reinforced plastics and similar durable materials are widely manufactured by three basic processes. In one process, liquid chemicals, commonly of polyisocyanurate formulation, are injected between the skins, after which they react and expand to form a rigid foam that bonds itself to the skins to form the sandwich panel. A second method of producing sandwich panels is by adhesive lamination wherein preformed panel skins are bonded by adhesive to cores of rigid foam boards or slabs that have been cut from expanded foam billets. In the third method, uncured resins and reinforcing materials are applied to the surfaces of such foam boards, or resins are introduced into closed or vacuum bagged molds containing the core and skin reinforcements and subsequently cured to form rigid skins. The curable resins may be, for example, thermosetting polyester, vinylester, epoxy, polyurethane or phenolic. Thermoplastic resins, such as polypropylene or polyetheretherketone (PEEK) may also be used, with the application of sufficient heat to cause them to flow and wet out the reinforcements. Reinforcements include such materials as glass, carbon or synthetic polymer fibers woven or stitched into fabrics or formed into dense mats of random fibers that are laid down in generally planar alignment.
Sandwich panel laminators use a wide variety of these preformed cores, including polyurethane, polyisocyanurate, extruded polystyrene, expanded polystyrene, polyvinylchloride and foam glass. Plastic foam cores for structurally demanding sandwich panel applications, such as the hulls of boats, are commonly made of linear or cross-linked polyvinyl chloride (PVC) formulations, in densities of from 2 to 16 pounds per cubic foot. The high cost of these materials per board foot has limited their use in such major median to high performance applications as highway trailers and recreational vehicles. A further drawback of the PVC foams and of other thermoplastic foams, such as polystyrene, is serious degradation of their physical properties at elevated temperatures encountered in transportation and other environments.
Plastic foam core sandwich panels often involve serious compromises in their design and cost due to inherent structural limitations of the rigid foam insulation cores. In addition to the deflection of these panels due to compressive and tensile stresses in the skins, further deflection results from the relatively low shear modulus of the rigid foam material. The thicker the core, the more important shear deflection becomes, to the point of exceeding deflection due to bending. Under a sustained load, the plastic foam core is also subject to creep deformation, further increasing panel deflection, with resulting risk of failure of the sandwich panel.
These deficiencies of the core may require increasing the strength and stiffness of the composite through the use of excessively heavy and expensive skins. Alternately, the panel could be improved structurally by increasing the thickness or density of the foam core beyond acceptable limits, which also raises the costs of both material and shipping. The relatively low compressive modulus of low density plastic foam cores also allows buckling of thin flat panel skins to occur at relatively low stress levels, again calling for overdesign of skins or higher density foam cores as a compensation. Low shear resistance and the absence of reinforcing elements within the foam core also permit the propagation under stress of cracks or fissures between the core and the panel skins as well as within or through the core itself, with resulting deterioration or structural failure of the panel. Still another difficulty is the low compressive strength of most plastic foams, which allows concentrated or impact loads to distort both skins and core.
Reinforcing frames or ribs of metal, wood, fiberglass reinforced plastic and other materials have been used in foam core sandwich panels to mitigate or overcome the structural limitations described above. Although both foam core and ribs contribute to the strength of these panels, the structural contribution of the ribs in such constructions is not fundamentally dependent upon the presence of the foam core.
An often serious drawback of widely spaced ribs is the creation of overly rigid sections of the structure within a generally more flexible panel. This can result in undesirable concentrated loads at the intersection of ribs and face laminates, especially with thinner face laminates made with higher strength composite materials. Structural properties of the composite may be improved by assembling between the skins a large number of individual blocks or strips of foam wrapped with fibrous reinforcing materials that connect the skins and fill the space between them. Impregnating resins are applied to both skin and core reinforcements during this layup process. Alternately, all components of skin and core reinforcement and foam may be positioned in a mold while in a dry and porous state, after which the mold is closed and resin is introduced under pressure, as in vacuum-assisted resin transfer molding, to flow into and impregnate the reinforcements.
Another common method of manufacturing a reinforced foam core utilizes adhering dry absorptive fibrous webs to alternating foam core panels. The reinforced foam core is provided by stacking rigid foam insulation boards and thin flexible fibrous sheets in alternating layers with adhesive between the layers, and then compressing the stack while the adhesive cures to form a core panel or billet. The billet is cut through the alternate layers and along parallel spaced planes to form reinforced foam core panels each having spaced webs formed by strips of the fibrous sheets. The method of using a reinforced foam core with the dry fibrous webs has become well-known in the industry as being desirable because the porosity in the dry webs allows for forming integral bonds by absorbing resin applied to the overlying panel skins.
Among the difficulties presented by known flooring assemblies are difficulty in the manufacture, assembly and installation of the flooring assemblies. The wood and steel flooring must be both manufactured and installed within the vehicle itself. The process does not allow for a cost saving prefabrication of floor sections.
Another difficulty presented by known flooring assemblies is the relatively high maintenance costs associated with such flooring. In this regard, known flooring assemblies are difficult to seal against moisture penetration. In the environment of mass transit vehicles, such as passenger trains, busses, and the like, moisture in the form of water and is often carried onto the flooring, and can seep into the flooring and into contact with the wood frame. Once the moisture is allowed to saturate the wood frame, the moisture is captured underneath the steel sheeting. This leads to an accelerated decomposition or rotting process. The rotted frame leaves the flooring susceptible to damage, which necessitates replacement of the rotted wood or replacement of the entire floor.
The present invention addresses these problems by providing a rugged, lightweight, water resistant composite flooring that is capable of simple manufacture and installation. This design uses proven materials and components, and features the superior flame and smoke performance ratings of phenolic composite materials. This design also offers a weight reduction compared to the traditional transit car floors.
In one embodiment, the invention provides a flooring assembly which is constructed of composite materials that are sufficiently rigid and water resistant so as to meet the rigors of the mass transit environment. In addition, the components of the flooring assemble are made of a composite material that pass the requisite safety testing.
In particular, the invention provides, among other things, a composite floor containing phenolic components. The flooring assembly includes a composite panel having a sandwich construction. The panel is made by bonding two thin skins, or facings to a relatively thick core material. The skins are made from a structural phenolic composite. The skins are cured after impregnating two fiberglass reinforcement sheets with phenolic resin. The core material includes a combination of lightweight and rigid closed-cell foam and phenolic ribs. The phenolic ribs provide the necessary reinforcement in the floor. The foam core resists moisture absorption and also provides superior bonding characteristics to the skins. Further advantages of the precured ribbed core and disadvantages of other core materials are further discussed below.
Phenolic closeout is molded into the edges of the floor panel to prevent moisture from entering the core and to provide extra strength and stability to the floor panel. The phenolic closeout surrounds the perimeter of the floor panel and is bonded by the same curing process to the top and bottom skin to seal the core from moisture.
The phenolic floor assembly is easily installed into the mass transit vehicles. The floor is assembled on site from multiple pre-fabricated floor panels. The phenolic closeout is easily machined, allowing the creation of a high density joint between panels. The pre-fabricated floor panels can be easily connected to each other by lap joints cut into the phenolic closeout. Tapping plates and mounting blocks, made from closeout material, can be cured within the core of the panel to provide extra support in mounting areas. These areas of the panel can easily be drilled and tapped on site to mechanically fasten the floor panel to the vehicle frame.
In one embodiment, the invention provides a flooring assembly including a plurality of floor panels. Each of the panels are interconnected and each panel includes a top skin defining a top surface, a bottom skin defining a bottom surface. The assembly also includes closeout member contacting the top skin and the bottom skin and defining in part the perimeter of said floor panel. The flooring assembly also includes a core located between the top and bottom skins. The core includes a ribbed core having a side wall contacting the closeout member.
In another embodiment, the invention provides a flooring assembly for use in passenger trains, wherein the passenger train includes a car assembly including spaced apart side walls, spaced apart end walls, and a floor support assembly. The flooring assembly includes multiple floor panels interconnected to cover the floor support assembly, the floor panels each including a top skin having two side edges, two end edges, an inner face, and an exposed face defining the top surface of said floor panel. Each floor panel also includes a bottom skin having two side edges, two end edges, an inner face, and an exposed face defining the bottom surface of said floor panel. Each floor panel also includes two side closeouts, each of the side closeouts including an inner face, an outer face exposed on the side of the floor panel, a top face contacting the inner face of the top skin, a bottom face contacting the inner face of the bottom skin, and two end faces defining the length of the side closeout. Each floor panel also includes two end closeouts including an inner face contacting the end faces of the side closeouts and defining with the side closeouts the outer perimeter of said floor panel. The flooring assembly also includes a core located within the side closeouts and end closeouts, the core includes side walls contacting the inner walls of the side closeouts and end closeouts. The core also includes a top surface contacting the inner face of the top skin, and a bottom surface contacting the inner face of the bottom skin.
Common core materials used in sandwich panels present many disadvantages. Among the difficulties presented by the reinforced foam cores with the dry webs is that the adhesives used to bond the dry webs to the foam cores prevent its use in industries with stringent safety standards. For example, the mass transit industry is regulated by numerous safety requirements, including flame and smoke tests for all flooring assemblies used in mass transit applications. The urethane adhesive, which is most commonly used for bonding, produces a gas when burned that does not meet the standards set forth by the regulations. Therefore, this type of reinforced core is unavailable for panels used within the mass transit industry.
Visual inspection of the quality of the phenolic bond in the ribs absent a destructive test can also be difficult or impossible. The dry webs become partially saturated from the liquid phenolic resin used to impregnate the glass fabric for the top and bottom skin. Because the web is cured at the same time as the top and bottom skin the integrity and extent of the curing that has taken place within the web is visually inaccessible. To determine the quality of the web cure, the web must in some way be made visually accessible. For example, the panel could be cut into a cross section.
Other difficulties presented by the adhered dry web foam core are inconsistent and non-uniform phenolic bonding due to the presence of adhesive and voids within the web. Ideally, the dry web material should be completely impregnated with the liquid phenolic resin to yield a consistent and uniform phenolic bond after curing. However, the urethane adhesive impregnates the dry material initially, which, in turn, does not allow for the proper impregnation of the phenolic resin. As a result, the urethane areas provide a weaker bond than the phenolic areas. Also, the adhesive is applied from a drip bar that drops adhesive at intermediate locations on the fabric surface. Because the adhesive is not evenly and completely applied to the web, air pockets and voids may occur during the resin impregnating stage. These voids in the web add to the uncertainty of the web characteristics. A fully phenolic bond produces a more structurally rigid and sound piece, which leads to consistent performance and reliability.
Another difficulty presented by the reinforced foam cores with the dry webs is the difficulty in handling because of the tenuous bond between the foam core and the dry web. The reinforced foam cores are used in the manufacturing of the phenolic panels and are therefore frequently handled and moved. This movement can often times result in broken bonds between the dry web and the foam core. This leads to either waste or extra cost to implement systems that will minimize breakage.
The present invention addresses these problems by providing a method of manufacturing a precured reinforced foam core that is capable of exceeding the mass transit industry""s safety standards and is capable of providing a visually accessible, stronger, and more uniform phenolic bond throughout the entire rib.
In particular, the precured reinforced core surpasses the safety tests required by the mass transit industry. The foam panels and glass fabric are bonded together using the liquid phenolic resin. The cured phenolic, along with the other materials making up the core, pass the requisite safety testing. Adhesives, which fail to meet the flame and smoke test safety standards, are not utilized.
Additionally, the quality of the phenolic bond is visually accessible before the core is hidden within the skins of the panel. After the bun is cured at a constant temperature and pressure, the bun is cut in a plane perpendicular to the plane of the foam sheets. The core strips will have two edges that will expose both the ribs and the foam. From this vantage point, the quality of the phenolic bond can be assessed visually. Also, the glass fabric does not contain any adhesives that prevent uniform saturation of the liquid phenolic resin. Because the quality of the phenolic bonds can be verified visually and uniform impregnation of the liquid phenolic resin can be achieved, the overall performance and quality of the floor panel will remain consistent.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.